Health experts on Thursday said more than 30 soil samples collected from service reservoirs in Sham Shui Po tested positive for a bacterium that causes melioidosis, but they have still been unable to determine what caused an outbreak in the area.

The Centre for Health Protection and the University of Hong Kong (HKU) made the conclusion as they wrapped up an epidemiology investigation in Sham Shui Po, following the emergence of a spate of melioidosis infections.

They found that 38 of the 471 environmental specimens collected in Sham Shui Po contained traces of Burkholderia pseudomallei – a bacterium which causes melioidosis – 32 of which were extracted from the soil in service reservoirs.

"Among the 32 positive soil samples from the service reservoirs... four have undergone genome sequencing. It was found that the genes of these four soil samples are very similar to that of the patients of the Sham Shui Po cluster. We believe there may be a connection," Albert Au from the Centre for Health Protection said, noting however that experts have yet to ascertain the cause of the outbreak.

Au said the government is amending legislation to list melioidosis as a statutory notifiable infectious disease, which will require doctors to report both suspected and confirmed cases.

Speaking at the same press conference, HKU top microbiologist Yuen Kwok-yung said global warming may be to blame for an increase in cases.

"We believe it's related to global warming. Under global warming, the amount of the bacterium in the soil will increase. And this kind of bacterium can survive both low and high temperatures. So we have grounds to believe that with the temperature rising globally, the risk of melioidosis infections will be increased, along with the amount of bacterium in the soil," he said.

Yuen advised Sham Shui Po residents not to use tap water that comes out within the first minute for showering, washing their face or brushing their teeth.

Melioidosis is transmitted through contact with contaminated soil or surface water, though human-to-human and person-to-person transmission is rare.

A total of 34 cases of melioidosis have been reported in Hong Kong so far this year – 20 of them from Sham Shui Po.